Resistance-capacitance coupled amplifier



- Feb. 12, 1 935. w. A.?F 1"rc1-| 1,990,781

RESISTANCE CAPACITANCE COUPLED AMPLIFIER 1 Original Filei Ap-ril 28, 1932 Inventor": William Arit m by His Amafney.

Patented Feb. 12, 1935 UNITED STATES RE SI STANCE-CAPACITANCE CCUPLED AMPLIFIER William A. Fitch, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 28, 1932, Serial No. 608,022

4 Claims. (Cl. 179171) My invention relates to amplifiers and more particularly to impedance or resistance coupled amplifiers for use over a wide range of audible and super-audible frequencies.

In television transmitters and receivers it is necessary to provide an amplifier having a. substantially fiat amplification characteristic for amplifying the frequency impulses which produce the television image. Such frequency impulses may range from an audio frequency of 20 cycles to a radio frequency of 200,000 cycles or more. It has been found that the ordinary resistance-capacitance coupled amplifier will amplify both audio and radio frequencies and this amplifier would be suitable for this purpose except for the fact that the amplification characteristic begins to droop appreciably in the neighborhood of 20,600 cycles and this drooping becomes more prominent as the frequency increases. To obtain the proper amplification of the higher frequencies which extend well into the radio. frequency range, several stages of amplification Would be necessary and the low frequencies would tend to overload the amplifier. The coupling capacitor between the respective electron discharge devices used in the amplifier tends to introduce a large phase shift at the lower frequencies, and this has the effect of shifting the various lines of the reproduced television image so that the picture becomes distorted.

In accordance with my invention, I provide a resistance-capacitance coupled amplifier with resistor and capacitor means in series with the coupling capacitor for obtaining substantially constant amplification throughout a wide range of audio and radio frequencies. If desired, an inductor may also be arranged in parallel with the resistor and capacitor means and connected in series with the normal coupling capacitor for neutralizing the phase shift introduced by the coupling capacitor.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization'and method of operation, together with other advantages thereof, will be better understood by referring to the following description taken in connection'with the accompanying drawing in which Fig. 1 illustrates an impedance coupled amplifier having a substantially fiat amplification characteristic for a range extending from low audio element may be biased negatively by means of a source of potential 2 connected to the cathode and the grid coupling resistor 3. Anode current is supplied to the electron discharge device 1 from a source of potential 4 which is connected to the anode through an anode coupling resistor 5. The anode of the electron discharge device 1 is coupled to the grid of the electron discharge device 6 by means of a coupling capacitor '7 The electron discharge device 6 is provided with a grid coupling resistor 8 a source of potential 9 for biasing the grid element. The anode of the electron discharge device 6 is supplied with current from a source. of potential 19 which is connected to the anode through the anode coupling resistor 11. The electron discharge device 6 may be coupled to an output circuit by means of the coupling capacitor 12. A conductor 13 provides a common connection to the cathodes of the electron discharge devices 1 and 6 and to the input and output circuits.

At resistor 14 is inserted in series with the coupling capacitor '7 to reduce the high amplification of the low audio frequency range. In order to prevent the resistor 14 from also reducing the amplification of the higher frequencies, particularly those which are in the super-audible range, a small by-pass capacitor 15 is connected in parallel with the resistor 14".. he capacitor 7 is of sufficient size so as to offer but a small impedance to the low audio frequencies which pass through this capacitor and through the resistor 14 where they are attenuated. The by-pass capacitor 15 offers a high impedance to the low audio frequencies so that substantially all of the low audio frequencies pass through the resistor 14. As thehigher frequencies appear across the output of the electron discharge device 1, the higher frequencies pass through the by-pass capacitor 15 as the by-pass capacitor offers a lower impedance to those frequencies than the resistor 14. Thus at the higher audio frequencies the by-pass capacitor l5 begins to pass a certain amount of the audio frequencies and as the upper limit of the radio frequency range is approached, the capacitor carries substantially the entire output of the electron discharge device 1. This arrangement of a resistor and by-pass capacitor in parallel connected in series with the couplingcapacitor then provides an amplifier having a substantially fiat amplification. characteristic for the entire range of frequencies.

Referring to Fig. 2, I have shown therein an amplifier of the resistance-capacitance coupled type which is similar to that shown in Fig. 1. Like parts in Fig. 2 are given the same reference numerals as in Fig. 1. When the amplifier shown in Fig. 2 is operated at the low audio frequency range, the coupling capacitor 7 introduces an appreciable phase shift. This phase shift is neutralized by providing an inductor 16 in parallel with the resistor l4 and the. capacitor 15. This inductor 16 is arranged to introduce an inductive reactive component which will compensate for the capacitive reactive component of the coupling capacitor '7 at a frequency approximating the lowest frequency at which. it is desired to a point somewhere near the beginning of the drooping portion of the amplification characteristic.

The' inductor 16 is arranged to resonate with the coupling capacitor 7 at a frequency approximating the lowest frequency for which the ameffect.

plifier is designed. The inductive reactive component introduced by. the inductor 16 exactly neutralizes the phase shift introduced by the .coupling capacitor '7 at the resonant frequency.

As the frequency increases the capacitive reactive component decreases and the phase shift introduced by the coupling capacitor becomes negligible at high frequencies. a At the same time, the inductivereactive component increases, but the inductor 16 is shunted by the resistor 14 and" the by-pass capacitor 15 which reduce the inductive In the lower'range of frequencies the signal load is carried by the resistor 14 which also tends to reduce, the normally high amplification or" the lower range of frequencies which may be considered to be audio frequencies. In the higher range of. frequencies the by-pass capacitor 15 carries the signal load. This arrangeinent'of the resistor 14, by-pass capacitor 15 and purpose of clearly setting forth the principles involved; It will, of course, be understood that I do .not wish to be limited thereto, since it is apparent that the principles herein disclosed are susceptible of numerous other applications, and modifications may be made in the circuit arrangements and in the instrumentalities employed without departing from thespirit and scope of my invention asset forth in the appended claims. a

What I claim as new and desire to secure by Letters Patent of the United States is:-

1. A. resistance-capacitance coupled amplifier arranged to operateover a wide range-of audio. and radio frequencies, said amplifier having a.

plurality of electrondischarge devices, acoupling capacitor connected between the anode of :one discharge device and the control element of a succeeding discharge device whereby the voltage supplied to said second discharge device is shifted in phase with respect to the output voltage of said first discharge device, an inductor in series with said coupling capacitor to neutralize said phase shift at substantially the lowest audio frequency to be amplified, means forrendering inappreciable the effect of said inductor throughout the remainder of the audio frequency range,

and means for rendering inappreciable the effect of said inductor and. said first means throughout the radio frequency range.

2. An impedance coupled amplifier arranged pacitor for neutralizing the phase shift of the coupling capacitor at substantially the lowest frequency to'be amplified.

3. In combination, a source of audio and radio frequency oscillations including. an electrondischarge device having an anode, a second electron discharge device having a grid circuit and an anode circuit, said grid circuit being arranged to repeat a wide range of audio and radio frequency oscillations from said source to said anode circuit, a coupling capacitor connected to said anode of said first electron discharge device, an inductor in series with said coupling capacitor to neutralize the reactance of said coupling capacitor at" substantially the lowest audio frequencyto be repeated, means in parallel with said inductor for shunting said inductor throughout the remainder'of the audio frequency range, and means in parallel with said inductor for shunting said inductor throughout the radiofrequency range.

4. A resistanceecapacitance coupled amplifier arranged to operate over awide range of audio and radio frequencies including a plurality of electron discharge devices each having a cathode, an anode anda controlelement, a coupling resistor connected between the anode and cathode of the first of said discharge devices, a coupling resistor connected between, the cathode and control element of the second of said discharge de-' vices, a coupling capacitor connected between the anode of one discharge device and the control memberof a succeeding discharge device, said capacitor normally introducing a phase shift between the voltages present across said coupling resistors, and said amplifier normally having a higher amplification characteristic at low frequencies than over the remainder of the operating frequency range, an inductor connected in series with said capacitor for neutralizing said phase shift. at substantially the lowest frequency to be amplified, a resistor connected in parallel to saidv inductor forreducing the amplification at audio frequencies and for rendering inappreciable the effect of said inductor throughout the higher audio frequency range, and a capacitor connected in parallel with said inductor and resistor for rendering inappreciablethe effect of said in ductor and resistor throughout the remainder of the frequency range.

7 WILLIAM A. FITCI-L 

